1. Field of the Invention
This invention relates to a refractometer used for measuring sugar concentration or density in a solution.
2. Description of Relevant Art
Refractometers used to measure sugar concentration or density in a solution by directing light on the boundary interface of a sample and a prism then detecting light reflected at the interface using a photoelectric sensor and measuring refractive index (sugar concentration or density) in the sample from the signal output by the photoelectric sensor are well-known. Measurements conducted using a refractometer operate on the principle that the critical angle of incidence at which total reflection occurs at the interface of a prism and a sample is dependent on refractive index of the sample.
A refractometer according to the prior art as disclosed in examined utility model application publication No. Hei 3-26443, generally comprises an optical system as shown in FIG. 1. In other words, between a prism 102 and a light source 104 is installed a condenser lens 106 for focusing light from the light source 104. Further, an objective lens 110 that focuses a beam output from the prism 102 over the photoelectric sensor 108 is installed between the prism 102 and a photoelectric sensor 108 to obtain a boundary position having a clear light/shade contrast.
There is a problem however as such optical systems require a plurality of optical elements and therefore incur high production costs. Further the elements used for construction that contain the optical elements must be arranged separately with respect to each other and therefore require very accurate positioning further contributing toward high production costs.
Normally, a refractometer is installed with a sample stage 114 surrounding the interface surface 112 that forms the interface between the prism 102 and a sample S. The sample stage 114 comes into contact with a variety of different sample materials that may be disposed thereon such as foods, chemicals, fats and oils, high molecular compounds and the like and is therefore made of a metal such as stainless steel that is highly resistant to corrosion.
After a measurement is performed the interface surface 112 and sample stage 114 to which the sample S has been applied must be wiped completely clean to ensure that none of the material from the previous sample remains to contaminate the next measurement. It is difficult to remove the sample material applied to the sample stage of a refractometer according to the prior art however, when the sample measured is a paste like substance such as starchy syrup or the like. Wiping away previous sample material takes time causing a problem of reducing the efficiency of measuring operations. Further, when the wiping action to remove sample material is repeated many times the sample stage 114 easily suffers abrasions.
A problem affecting refractometers used to measure highly corrosive sample substances such as battery fluid or the like is that the sample stage 114 has a very short usable life. Further, if a sample such as an adhesive or the like that adheres strongly to the sample stage 114 is used it may not be possible to strip the sample material away thereby rendering further measurements impossible.
A refractometer operates on the premise that only reflected light of the interface surface 112 enters the photoelectric sensor 108, however a refractometer is not restricted to being used only indoors. When measuring samples such as a juice extract from a fruit or vegetable or antifreeze solution used in an automobile for example, the refractometer may frequently be used outdoors. In these situations external light rays that are changeable in space and time transmit the prism 102 from the direction of the sample itself and enter the photoelectric sensor 108. Thus, another problem affecting refractometers is that refractive index cannot be accurately measured in outdoor environments.
In order to enable refractive index to be accurately measured when outdoors, the user may use their hand as a cover from above the sample S or block the external light rays by using a cover, however when taking a measurement, using a hand as a cover or opening and closing a cover is troublesome and prevents measurements from being performed efficiently.